a) Field of the Invention
The invention relates to a method for controlling the progressive charge of a motor vehicle alternator, which is designed to produce an electrical supply voltage for the on-board network of the latter, in response to an excitation current which is applied to the alternator.
b) Description of the Prior Art
Methods for controlling the progressive charge of a motor vehicle alternator are already known.
The progressive charge function in an alternator makes it possible in particular to avoid stalling the thermal engine when the latter is functioning at low speed, particularly when idling, and substantial electric charges are activated in the vehicle.
The progressive charge function prevents stalling of the thermal engine by avoiding a rapid increase in the torque which is collected on the thermal engine by the alternator if a substantial electric charge is triggered. For this purpose, significant variations of the excitation current applied to the alternator rotor are limited by the progressive charge function.
For example, taking into consideration a variation of electric charge of 50 Amps with 15 Volts, i.e. 750 Watts, with an alternator output of 50%, the variation of mechanical power which is collected on the engine is 1500 Watts, and represents approximately 13% of the power supplied at idling speed by a thermal engine with average power. In certain cases, a variation of charge of this type can give rise to stalling of the thermal engine.
FIGS. 1A and 1B show the advantages and disadvantages provided by the progressive charge function in control of the alternator. These figures correspond to a situation in which the alternator functions at a constant speed, in an alternator speed range in which the progressive charge function is activated, i.e. a speed of between NALT=0 rpm at NALT=NALT-S. Beyond the speed NALT-S, for example NALT-S=3000 rpm (i.e. an engine speed of approximately 1000 rpm), the progressive charge function is inhibited, since the thermal engine then has sufficient power to eliminate any risk of stalling.
FIG. 1A shows the effects on the torque ΓALT collected by the alternator on the thermal engine, and on the regulated voltage UBA supplied by the alternator, for example during a variation of electric charge L30/90 on an alternator, i.e. from 30% to 90% of the nominal power of the alternator, in an alternator which is not equipped with the progressive charge function.
As shown in FIG. 1A, when the variation of charge L30/90 takes place, a loop for regulation of the voltage UBA which is provided in the alternator gives rise to a strong increase in the cyclical ratio DCEXC of an excitation current lexc of the PWM (Pulse Width Modulation) type, which supplies an excitation coil of the alternator rotor.
In the example in FIG. 1A, this cyclical ratio goes from 40 to 80%, passing via a peak of 100%, and the continuous mean value (shown in FIG. 1A) of the excitation current lexc is subjected to a sudden variation which affects the torque ΓALT which is collected by the alternator on the thermal engine. A sudden variation of this type can give rise to stalling of the thermal engine.
On the other hand, in its progressive charge function, the regulation loop functions fully in order to maintain the voltage UBA at its set value. This results in low variation Δ1UBA of the voltage UBA as a result of the strong variation allowed for the excitation current lexc and consecutively for the torque ΓALT collected.
FIG. 1B shows the effects on the torque ΓALT collected by the alternator on the thermal engine and on the regulated voltage UBA supplied by the alternator, for example during variation of electric charge L30/90 on an alternator, i.e. from 30% to 90% of the nominal power of the alternator, in an alternator which is equipped with the progressive charge function.
As shown in FIG. 1B, when the variation of electric charge L30/90 occurs, the variation of the cyclical ratio DCEXC of 40 to 80% takes place “gradually”, as a result of the action of the progressive charge function. Consequently, the excitation current lexc and the torque ΓALT vary progressively, and the thermal engine is not acted upon suddenly, thus minimising the risk of stalling of the thermal engine.
On the other hand, with the progressive charge function, as a result of the limitation of the variation of the cyclical ratio DCEXC, the voltage regulation loop UBA is “curbed”, and substantial variation Δ2UBA of the voltage UBA occurs during the variation of the charge L30/90.
This substantial variation of the voltage UBA is one of the known disadvantages of the progressive charge function, and can give rise to certain problems, in particular in situations of periodic variations of the alternator charge.
For example, a situation of periodic variation of the alternator charge takes place when the emergency lights of the vehicle are activated. These period variations of the alternator charge introduce a periodic variation of the voltage UBA, and consecutively, for example, visual disturbance in the light beams of the lighting headlights of the vehicle when these are activated.
Particular arrangements are known in the prior art for control of the progressive charge function, in situations of periodic switching of the alternator charge.
In general, these solutions according to the known art use an intermediate signal known as PCR (Progressive Charge Return), which defines a maximum authorised value of the cyclical ratio DCEXC. During increasing charge switching, the value of the cyclical ratio DCEXC is updated to the value of the PCR signal. The development of the progressive charge return signal PCR is generated over a period of time by the progressive charge function. This control permits certain jumps in the cyclical ratio DCEXC for increasing charge switchings which take place after a first increasing charge switch, such as to limit subsequent variations of voltage which take place after the first variation of voltage Δ1UBA.
FIGS. 2A and 2B show functioning of the progressive charge in a situation in which the speed NALT of the alternator drops, and goes from a speed N1ALT for which the progressive charge function is not active, to a speed N2ALT which is lower than the speed NALT-S, and for which the progressive charge function is triggered, and limits the cyclical ratio DCEXC. A situation of this type occurs for example when the vehicle slows down when a braking operation takes place.
As shown in FIG. 2A, the increase in the cyclical ratio DCEXC commanded by the regulation loop, consecutively to the drop in speed and the drop in the correlative voltage, is limited according to the curve portion B by the progressive charge function, which leads to a significant drop Δ21UBA in the voltage UBA. Without the action of the progressive charge function, the increase in the cyclical ratio DCEXC commanded by the regulation loop would have developed according to the portion of curve A, and would have led to a substantially smaller drop in the voltage UBA.
FIG. 2B shows the behaviour of the alternator when the progressive charge function comprises the use of an intermediate PCR signal which defines a maximum permitted value of the cyclical ratio DCEXC.
As shown in FIG. 2B, in a situation of this type, for as long as the speed of rotation NALT=N1ALT is greater than the threshold speed NALT-S, the cyclical ratio DCEXC authorised by the intermediate PCR signal is at its maximum value, i.e. 100%.
When the speed NALT drops below the threshold NALT-S, a linear decrease (portion indicated as C) of the intermediate PCR signal, according to a gradient which is predetermined by the progressive charge function, is commanded such as to impose an upper limit on the development of the cyclical ratio DCEXC. In the known solutions, this decrease in the PCR signal continues until the effective value of the cyclical ratio DCEXC is stabilised. Thus, when an additional charge demand occurs after this stabilisation, no jump in the cyclical ratio DCEXC should be authorised, since the maximum value of the cyclical ratio DCEXC provided by the PCR signal is equal to the present value of the cyclical ratio DCEXC. The cyclical ratio DCEXC can then increase only according to a determined progressive charge gradient which is imposed by the PCR signal, in order to avoid a sudden increase in the torque ΓALT.
FIG. 2B shows a possible situation with solutions according to the prior art, according to which an increase D in the electrical charge takes place after stabilisation (at 70% in this example) of the cyclical ratio DCEXC whereas the value (85% in this example) of the PCR signal has not yet reached the stabilised value (70%) of the cyclical ratio DCEXC. In such a case, the progressive charge function authorises a jump E in the cyclical ratio DCEXC, taking into account the fact that the intermediate PCR signal has a value (85% in this example) which is greater than the stabilised value (70%) of the cyclical ratio DCEXC, at the instant when the increase D in the electrical charge takes place. In this example, the value of the cyclical ratio DCEXC goes from 70% to 80% and is then stabilised at that value.
In this situation, as shown in FIG. 2B, the torque ΓALT undergoes a sudden increase F which can give rise to stalling of the thermal engine, whereas the progressive charge function is active.
More generally, the above-described situation of a sudden increase F could also occur in cases of variations of the engine speed which take place within the alternator speed range in which the progressive charge function is activated, without requiring a return of the speed NALT into the range of activation of the progressive charge, as in the preceding example.
It therefore appears necessary to propose an improvement in the progressive charge function in an alternator, such as to avoid possible stalling of the thermal engine, in the situation in which an increase in the electrical charge occurs whilst the progressive charge intermediate return signal has not yet reached a stabilised value of the cyclical ratio of the alternator excitation current.